1. Field of the Invention
The present invention relates to thermal inkjet printer technology. More specifically, the present invention relates to systems and techniques for energizing heater resistors within an inkjet printhead to expel ink.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Thermal inkjet printers are currently used for a wide variety of high speed, high quality printing applications. These printers include a thermal inkjet printhead. The thermal inkjet printhead includes one or more ink-filled channels communicating with an ink supply chamber or cartridge at one end and having an opening at the opposite end, referred to as a nozzle. A heater resistor is located in the channel at a predetermined distance underneath the nozzle. The resistors are individually addressed with a current pulse to momentarily vaporize the ink to form a bubble. The bubble expels an ink droplet towards a recording medium such as paper. By energizing heater resistors in different combinations as the printhead moves across the paper, an inkjet printer prints different characters on the paper.
The heater resistors within the printhead are addressed through flexible conductors that connect the resistors to control circuitry within the thermal inkjet printer. In many prior systems, each resistor was connected directly to a flexible conductor. However, inasmuch as resolution of the printed characters is improved by adding nozzles, the drive for greater print quality has created an associated increase in the number of heater resistors in a printhead. This caused an associated increase in the number of conductors required to address the individual heater resistors. To minimize the number of conductors required, many resistors were connected to a common return line. Thus, the conventional printhead had one conductor per resistor and a common return.
With as many as 10-13 resistors per common return, the cumulative current in the return was, in many cases, so high as to cause a significant voltage drop and associated power dissipation in the return line. This lowered the voltage and power delivered to the heater resistor. Hence, because of the resistance of the power and return conductors of a thermal inkjet printhead, the power delivered to the individual elements was a function of the number of the elements energized. Since, optimum print quality requires precise control of the energy supplied to the heater resistor, losses in the return line were adversely affecting the operation of the system.
This effect was minimized by energizing only one element per power/return pair. In these systems, external power transistors were activated in sequence to provide drive current to the heater resistors to be fired during a print cycle.
However, the provision of a separate transistor per resistor was expensive. In addition, this technique required a large number of external connections to the printhead and a considerable amount of power was lost in the control element used to sequence the transistors.
The interconnect problem was mitigated somewhat by numerous decoding schemes. One such scheme is that of U.S. Pat. No. 3,852,563, entitled THERMAL PRINTING HEAD, issued Dec. 3, 1974 to J. H. Bohorquez, the teachings of which are incorporated herein by reference.
A more sophisticated multiplexing scheme was developed by which logic circuitry comprising active elements (transistors) were added to the printhead.
In any event, the loss elements were the trace (the conductor from the resistor to the contact to the external circuitry), the heating element, and the return are all loss elements. Nonetheless, a problem remained in delivering a correct voltage to the heating element notwithstanding changes in the circuitry surrounding the element.
U.S. Pat. No. 5,083,137 entitled ENERGY CONTROL CIRCUIT FOR A THERMAL INK-JET PRINTHEAD, issued Jan. 21, 1992 to Badyal et al. (the teachings of which are also incorporated herein by reference) discloses a system for addressing the problem by controlling the power to each heating element individually. A measurement resistor is added and used to measure the current through the heater resistor. By regulating the power delivered to the element, the energy may be delivered to the element independent of the losses in the power and return lines.
However, this method has several disadvantages, First, a considerable amount of additional circuitry is required in order to control the current through each heater resistor. This is costly in manufacturing time and space on the substrate. In addition, the measurement resistor and the other control elements are lossy.
Thus, a need remains in the art for a more efficient, less expensive technique for individually controlling the power applied to a heater resistor in the printhead of an inkjet printer.